Pressure sensor assembly for wellbore

ABSTRACT

In the context of a casing-conveyed downhole pressure sensor assembly, there is provided a perforating charge which can be detonated from ground surface to open a perforation providing a pathway between the exterior formation and the pressure sensor.

FIELD OF THE INVENTION

[0001] The present invention relates to a casing—conveyed sensor assembly for use in a wellbore to measure a subterranean formation condition, such as pressure.

BACKGROUND OF THE INVENTION

[0002] Casing—conveyed downhole pressure sensors are presently used in the oil/gas industry, to monitor pressure in a subterranean formation. A conventional downhole pressure monitoring system comprises:

[0003] a pad or housing which is secured at ground surface to the exterior surface of a joint of tubular casing—the joint is run into a wellbore as part of a casing string with the result that the housing is located opposite the subterranean formation whose pressure is to be monitored;

[0004] the side wall of the housing forms an interior chamber in which is mounted the pressure sensor;

[0005] a cable extends down the wellbore and is secured to the outside surface of the casing string—wiring in the cable is connected to the sensor, for providing power to it and conveying signals indicative of pressure measurement to recording means at ground surface;

[0006] the sensor is sealed within the sensor chamber by suitable seal means, to keep it dry; and

[0007] a pressure port or passageway extends from the sensor chamber through the housing side wall to the exterior surface of the housing—the port and sensor chamber are packed with a filling of oil and grease, which function to isolate the sensor means from wellbore or formation fluid and which act as a pressure transmission medium.

[0008] The casing string is cemented in place within the wellbore. As a result, a layer of cement fills the annular space defined between the casing string and the wellbore. The layer of cement is usually sufficiently porous and thin enough so that formation fluid pressure can penetrate therethrough to reach and pressurize the pressure transmission medium. As a result, measurements of formation pressure can be taken by the sensor.

[0009] However, in some cases it has been found that formation pressure does not reach the sensor. This may be due to invasion by the cement out into the formation or plugging of the pressure port with cement or an impermeable drilling fluid filter cake. It therefore is desirable to provide a sensor assembly adapted to overcome this problem, should it arise.

SUMMARY OF THE INVENTION

[0010] The present invention is preferably concerned with a pressure sensor assembly for use downhole in a wellbore as part of a pressure monitoring system. A preferred embodiment of the sensor assembly comprises:

[0011] housing means for conveyance on well casing, the housing means forming a sensor chamber and a separate charge chamber;

[0012] a pressure sensor positioned in the sensor chamber, operative to measure formation pressure for transmittal to ground surface, for example through a monitoring system cable;

[0013] a shaped explosive perforating charge positioned in the charge chamber. The perforating charge can be detonated from ground surface using means such as suitable wiring incorporated in the monitoring system cable;

[0014] the housing means forms a passageway, containing pressure transmission medium, connecting the sensor with the exterior surface of the housing means; and

[0015] the passageway is positioned in the path of the jet produced by detonating the perforating charge.

[0016] As a result of this combination, if desired one can detonate the perforating charge to produce a jet which creates a perforation extending through the housing side wall and the cement layer to penetrate the formation. This perforation will cross or intersect the pressure transmission passageway. As a result, formation fluid may enter through the jet perforation and reach the passageway to apply its pressure to the sensor through the pressure transmission medium.

[0017] A single housing may be used to form both the sensor and perforating chambers. Alternatively, the perforating chamber and part of the pressure transmission passageway can be provided in a second housing, forming part of the housing means.

[0018] While the invention is described in connection with a pressure sensor, it will be understood that it can be applied with other instrumentation for measuring another formation condition, such as temperature. Therefore the words “formation condition measuring”, used to modify “sensor” or “sensor assembly”, are to be widely interpreted.

[0019] In accordance with the invention, therefore, a casing-conveyed condition measuring sensor, contained in a protective housing means, is combined with selectively activatable means for forming a perforation (for example, a shaped explosive perforating charge). The perforation-forming means is oriented and capable of forming an outwardly directed perforation extending through the cement layer and into the formation. In addition it is selected so as to form a perforation adapted to enable formation pressure or the like to reach the sensor. The perforation-forming means may be contained within the housing means or may be external of it.

[0020] Broadly stated, in one embodiment the invention is concerned with a downhole formation condition measuring sensor assembly for conveyance on the exterior surface of a string of casing to a subterranean formation to measure a condition in the formation, comprising: housing means, for attachment to the exterior surface of the casing string, forming a sensor chamber; a sensor, mounted within the sensor chamber, for measuring the formation condition; and an explosive perforating charge, associated with the housing means and oriented to form an outwardly extending perforation, when detonated, for penetrating the formation and enabling transmission of the condition from the formation to the sensor.

[0021] In another embodiment, the invention is concerned with a method for connecting a subterranean formation with a formation condition measuring sensor, comprising: providing a downhole sensor assembly, externally conveyed on a string of casing cemented in a wellbore so that the sensor assembly is located opposite the formation, the sensor assembly comprising housing means containing a sensor for measuring the formation condition and means for forming, when activated, an outwardly directed perforation penetrating into the formation and operative to enable the formation condition to reach the sensor; and activating the perforation-forming means from ground surface, when required, to form the perforation.

DESCRIPTION OF THE DRAWING

[0022]FIG. 1 is a schematic side view showing a wellbore, cemented casing string, and casing-conveyed downhole pressure sensor assembly positioned opposite a subterranean formation;

[0023]FIG. 2 is a partly sectional side elevation showing a perforating charge in a separate lower housing; and

[0024]FIG. 3 is a partly sectional side elevation showing a pressure sensor in a separate upper housing, for use with the assembly of FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0025] The invention involves the concept of:

[0026] providing perforation-forming means, such as a shaped explosive perforating charge, in association with a downhole pressure sensor assembly having housing means containing a sensor in a sensor chamber, the sensor assembly being mounted to the exterior surface of a casing string, often cemented, in a wellbore penetrating a subterranean formation; and

[0027] activating the perforation-forming means from ground surface, when required, to form a perforation extending into the formation, the perforation being in communication with the sensor to enable pressure transmission between the formation and the sensor.

[0028] FIGS. 1-3 show a preferred embodiment for implementing the invention. However, it will be apparent to those skilled in the art that various alternatives can be used to implement the steps just described.

[0029] In this embodiment, a downhole pressure sensor assembly 1 is provided. The sensor assembly 1 will have been secured to the exterior surface 2 of a casing joint 4 forming part of a casing string 5, prior to running the string into a wellbore 6. The casing string 5 is cemented in place in the wellbore 6. The sensor assembly 1 is positioned opposite a subterranean formation 7, whose pressure is to be monitored. A layer 8 of cement surrounds the casing joint 4 and sensor assembly 1. A cable 9 extends from ground surface 10 along the outer surface of the casing string 5. The cable 9 is operatively connected with the sensor assembly 1 to power the electronics (not shown) of a conventional pressure sensor 11 and convey pressure measurement signals back to a data recorder 12 at ground surface 10.

[0030] The sensor assembly 1 comprises a charge housing 13 formed by a bottom protector nose sub 14, a standoff pressure port sub 15 and a top protector nose sub 16. The subs 14, 15, 16 are connected together by socket bolts 17.

[0031] The side wall 18 of the port sub 15 forms an internal perforating charge chamber 19. A charge carrier 20 is positioned in the chamber 19. This charge carrier 20 has a locator sub 21 at its base which includes a pin 22, which seats in a pin hole 23 formed in the bottom wall 24 of the port sub 15 to orient the carrier 20. One or more shaped explosive perforating charges 25 are carried by the carrier 20. A conductor 26 from the cable 9 connects to a firing boot 27, for activating or detonating the detonator 3 of the charge 25. The carrier 20 is suspended at its upper end from a connector insulator block 28 which closes the top end of the charge chamber 19. The connector insulator block 28 is sealed by O-rings 29 with the interior surface 30 of the port sub 15. The connector insulator block 28 forms an axial bore 32. A top connector block 33, having an axial bore 34, is threaded into the upper end of the bore 32. The conductor 26 extends through the axial bores 32, 34 and continues through the cable 9 to ground surface 10.

[0032] The side wall 18 of the port sub 15 also forms a pressure port or passageway 35 having horizontal and vertical sections 36, 37. The pressure passageway radial section 36 terminates in an opening at the outer surface 39 of the port sub 15. A connector 40 is screwed into the upper end 41 of the port vertical section 37. The connector 40 connects with a conduit 42 extending up through a top aperture 43 formed through the top nose sub 16.

[0033] The sensor assembly 1 further comprises a sensor housing 50 formed by a bottom sub 51, sensor sub 52 and top sub 53, connected together by socket bolts 54.

[0034] The sensor sub 52 forms a chamber 55 in which is positioned a pressure sensor 11.

[0035] The sensor 11 is connected at its upper end by wiring 57 through a bulkhead 60 to the cable clamp 58 of the cable 9. Suitable wiring (not shown) forming part of the cable 9 provides connection between the sensor wiring 57 and recording means 12 at ground surface.

[0036] At its lower end, the sensor 56 is connected through a bulkhead 58 with the conduit 42 extending from the pressure passageway 35.

[0037] Thus, formation pressure, transmitted through the pressure passageway 35 and conduit 42 and their contained grease and silicon oil medium 59, reaches the sensor 56 under normal operation. However, in the event the formation pressure is not being transmitted through the pressure passageway 35, a perforating charge 25 can be detonated to form a perforation intersecting passageway 35 and penetrating through the cement layer 8 into the formation 7.

[0038] It will be appreciated by those skilled in the art that downhole perforating charge assemblies and pressure-measuring sensor assemblies are conventional and require no detailed description. It is their combination and the orientation of the charge and pressure passageway that come together to yield the desired objective. 

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:
 1. A downhole pressure sensor assembly for conveyance on the exterior surface of a string of casing to a subterranean formation to measure pressure in the formation, comprising: housing means, for attachment to the exterior surface of the casing string, forming a sensor chamber; a sensor, mounted within the sensor chamber, for measuring formation pressure; and a shaped explosive perforating charge, associated with the housing means and oriented to form an outwardly extending perforation, when detonated, for penetrating the formation and enabling transmission of pressure from the formation to the sensor.
 2. The downhole pressure sensor assembly as set forth in claim 1 wherein: the perforating charge is contained within the housing means; the housing means has an external surface and forms a passageway, connecting the external surface with the sensor, for transmitting formation pressure to the sensor; and the charge is oriented so that the perforation will intersect the passageway when the charge is detonated.
 3. The downhole sensor assembly as set forth in claim 2 wherein: the housing means forms a second chamber, separate from the sensor chamber, wherein the perforating charge is positioned.
 4. A method for connecting a subterranean formation with a sensor for measuring a formation condition, comprising: providing a downhole sensor assembly, externally conveyed on a string of casing cemented in a wellbore so that the sensor assembly is located opposite the formation, the sensor assembly comprising housing means containing a sensor for measuring the formation condition and means for forming, when activated, an outwardly directed perforation penetrating into the formation for enabling the formation condition to reach the sensor for measurement thereof; and activating the perforation-forming means from ground surface, when required, to form the perforation.
 5. A method for connecting a subterranean formation with a pressure sensor, comprising: providing a downhole pressure sensor assembly, externally conveyed on a string of casing cemented in a wellbore so that the sensor assembly is located opposite the formation, the sensor assembly comprising housing means containing a pressure sensor and means for forming, when activated, an outwardly directed perforation, the housing means having an external surface and forming a passageway connecting the sensor with the housing means external surface, the perforation-forming means being oriented so that the perforation, when formed, will intersect the passageway and penetrate into the formation so that formation pressure may be transmitted through the perforation and passageway to the sensor; and activating the perforation-forming means from ground surface, when required, to form the perforation.
 6. A method for connecting a subterranean formation with pressure sensor means comprising: providing perforation-forming means in association with a casing-conveyed downhole pressure sensor assembly, said sensor assembly having housing means containing pressure-measuring sensor means and being mounted to the exterior surface of a cemented casing string in a wellbore penetrating the formation; and activating the perforation-forming means from ground surface to form a perforation extending into the formation, the perforation being in communication with the sensor means to enable pressure transmission between the formation and the sensor means.
 7. A downhole formation condition measuring sensor assembly for conveyance on the exterior surface of a string of casing to a subterranean formation to measure a condition in the formation, comprising: housing means, for attachment to the exterior surface of the casing string, forming a sensor chamber; a sensor, mounted within the sensor chamber, for measuring the formation condition; and an explosive perforating charge, associated with the housing means and oriented to form an outwardly extending perforation, when detonated, for penetrating the formation and enabling transmission of the condition from the formation to the sensor. 